These receivers were originally built as Model 1928A1’s, but were apparently either pulled from production, or assembled with remaining receivers for domestic sales to police and other civilian entities during and after WWII. Thompsons such as this did not have military inspector markings added, and since they were not destined to the military, they had the “U.S.,” and the “1” in “28A1” ground off the receiver markings. The “1” was replaced with a “C,” which indicates the presence of a Cutts compensator, utilizing the original Auto-Ordnance nomenclature for the model type. Thus, the model is known as a “Model 1928AC.” Many examples of this lesser encountered Thompson variation, including this one, had an “X” suffix added to the serial number. Examples of these were sold by Auto-Ordnance as late as 1946, and were put together with spare parts. Many have smooth barrels like this one, which is technically correct for the model, based on the available factory parts of the time. This one also has pin-style selector levers, which is consistent with later Thompson production. This Thompson is a variation that many collectors do not have in their collection, and would be prized by a beginner, or an advanced collector.
We have had 2 of these variations recently, about 3000 serials away from each other, just in case you might think the same one was posted twice.